Internet Engineering Task Force (IETF) B. Ver Steeg
Request for Comments: 6285 A. Begen
Category: Standards Track Cisco
ISSN: 2070-1721 T. Van Caenegem
Alcatel-Lucent
Z. Vax
Magnum Semiconductor
June 2011
Unicast-Based Rapid Acquisition of Multicast RTP Sessions
Abstract
When an RTP receiver joins a multicast session, it may need to
acquire and parse certain Reference Information before it can process
any data sent in the multicast session. Depending on the join time,
length of the Reference Information repetition (or appearance)
interval, size of the Reference Information, and the application and
transport properties, the time lag before an RTP receiver can
usefully consume the multicast data, which we refer to as the
Acquisition Delay, varies and can be large. This is an undesirable
phenomenon for receivers that frequently switch among different
multicast sessions, such as video broadcasts.
In this document, we describe a method using the existing RTP and RTP
Control Protocol (RTCP) machinery that reduces the acquisition delay.
In this method, an auxiliary unicast RTP session carrying the
Reference Information to the receiver precedes or accompanies the
multicast stream. This unicast RTP flow can be transmitted at a
faster than natural bitrate to further accelerate the acquisition.
The motivating use case for this capability is multicast applications
that carry real-time compressed audio and video. However, this
method can also be used in other types of multicast applications
where the acquisition delay is long enough to be a problem.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
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RFC 6285 RAMS June 2011
Information is sent in the session. In this case, the required
waiting time is usually minimal. Other times, the receiver might
join the session right after the Reference Information has been
transmitted. In this case, the receiver has to wait for the
Reference Information to appear again in the flow before it can start
processing any multicast data. In some other cases, the Reference
Information is not contiguous in the flow but dispersed over a large
period, which forces the receiver to wait for the whole Reference
Information to arrive before starting to process the rest of the
data.
The net effect of waiting for the Reference Information and waiting
for various buffers to fill up is that receivers can experience
significantly large delays in data processing. In this document, we
refer to the difference between the time an RTP receiver wants to
join the multicast session and the time the RTP receiver acquires all
the necessary Reference Information as the Acquisition Delay. The
acquisition delay might not be the same for different receivers; it
usually varies depending on the join time, length of the Reference
Information repetition (or appearance) interval, and size of the
Reference Information, as well as the application and transport
properties.
The varying nature of the acquisition delay adversely affects the
receivers that frequently switch among multicast sessions. While
this problem equally applies to both any-source multicast (ASM) and
source-specific multicast (SSM) applications, in this specification
we address it for the SSM-based applications by describing a method
that uses the fundamental tools offered by the existing RTP and RTCP
protocols [RFC3550]. In this method, either the multicast source (or
the distribution source in an SSM session) retains the Reference
Information for a period after its transmission, or an intermediary
network element (that we refer to as Retransmission Server) joins the
multicast session and continuously caches the Reference Information
as it is sent in the session and acts as a feedback target (see
[RFC5760]) for the session. When an RTP receiver wishes to join the
same multicast session, instead of simply issuing a Source Filtering
Group Management Protocol (SFGMP) Join message, it sends a request to
the feedback target for the session and asks for the Reference
Information. The retransmission server starts a new unicast RTP
(retransmission) session and sends the Reference Information to the
RTP receiver over that session. If there is residual bandwidth, the
retransmission server might burst the Reference Information faster
than its natural rate. As soon as the receiver acquires the
Reference Information, it can join the multicast session and start
processing the multicast data. A simplified network diagram showing
this method through an intermediary network element is depicted in
Figure 1.
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This method potentially reduces the acquisition delay. We refer to
this method as Unicast-Based Rapid Acquisition of Multicast RTP
Sessions. A primary use case for this method is to reduce the
channel-change times in IPTV networks where compressed video streams
are multicast in different SSM sessions and viewers randomly join
these sessions.
-----------------------
+--->| Intermediary |
| | Network Element |
| ...|(Retransmission Server)|
| : -----------------------
| :
| v
----------- ---------- ----------
| Multicast | | |---------->| Joining |
| Source |------->| Router |..........>| RTP |
| | | | | Receiver |
----------- ---------- ----------
|
| ----------
+---------------->| Existing |
| RTP |
| Receiver |
----------
-------> Multicast RTP Flow
.......> Unicast RTP Flow
Figure 1: Rapid Acquisition through an Intermediary Network Element
A principle design goal in this solution is to use the existing tools
in the RTP/RTCP protocol family. This improves the versatility of
the existing implementations and promotes faster deployment and
better interoperability. To this effect, we use the unicast
retransmission support of RTP [RFC4588] and the capabilities of RTCP
to handle the signaling needed to accomplish the acquisition.
A reasonable effort has been made in this specification to design a
solution that reliably works in both engineered and best-effort
networks. However, a proper congestion control combined with the
desired behavior of this solution is difficult to achieve. Rather,
this solution has been designed based on the assumption that the
retransmission server and the RTP receivers have some knowledge about
where the bottleneck between them is. This assumption does not
generally hold unless both the retransmission server and the RTP
receivers are in the same edge network. Thus, this solution should
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not be used across any best-effort path of the Internet.
Furthermore, this solution should only be used in networks that are
already carrying non-congestion-responsive multicast traffic and have
throttling mechanisms in the retransmission servers to ensure the
(unicast) burst traffic is a known constant upper-bound multiplier on
the multicast load.
1.1. Acquisition of RTP Streams vs. RTP Sessions
In this memo, we describe a protocol that handles the rapid
acquisition of a single multicast RTP session (called a primary
multicast RTP session) carrying one or more RTP streams (called
primary multicast streams). If desired, multiple instances of this
protocol may be run in parallel to acquire multiple RTP sessions
simultaneously.
When an RTP receiver requests the Reference Information from the
retransmission server, it can opt to rapidly acquire a specific
subset of the available RTP streams in the primary multicast RTP
session. Alternatively, the RTP receiver can request the rapid
acquisition of all of the RTP streams in that RTP session.
Regardless of how many RTP streams are requested by the RTP receiver
or how many will be actually sent by the retransmission server, only
one unicast RTP session will be established by the retransmission
server. This unicast RTP session is separate from the associated
primary multicast RTP session. As a result, there are always two
different RTP sessions in a single instance of the rapid acquisition
protocol: (i) the primary multicast RTP session with its associated
unicast feedback and (ii) the unicast RTP session.
If the RTP receiver wants to rapidly acquire multiple RTP sessions
simultaneously, separate unicast RTP sessions will be established for
each of them.
1.2. Outline
The rest of this specification is as follows. Section 3 provides a
list of the definitions frequently used in this document. In
Section 4, we describe the delay components in generic multicast
applications. Section 5 presents an overview of the protocol design
considerations for rapid acquisition. We provide the protocol
details of the rapid acquisition method in Sections 6 and 7.
Sections 8 and 9 discuss the Session Description Protocol (SDP)
signaling issues with examples and NAT-related issues, respectively.
Finally, Section 10 discusses the security considerations, and
Section 11 details the IANA considerations.
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RFC 6285 RAMS June 20112. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Definitions
This document uses the following acronyms and definitions frequently:
(Primary) SSM Session: The multicast session to which RTP receivers
can join at a random point in time. A primary SSM session can carry
multiple RTP streams.
Primary Multicast RTP Session: The multicast RTP session an RTP
receiver is interested in acquiring rapidly. From the RTP receiver's
viewpoint, the primary multicast RTP session has one associated
unicast RTCP feedback stream to a Feedback Target, in addition to the
primary multicast RTP stream(s).
Primary Multicast (RTP) Streams: The RTP stream(s) carried in the
primary multicast RTP session.
Source Filtering Group Management Protocol (SFGMP): Following the
definition in [RFC4604], SFGMP refers to the Internet Group
Management Protocol (IGMP) version 3 [RFC3376] and the Multicast
Listener Discovery Protocol (MLD) version 2 [RFC3810] in the IPv4 and
IPv6 networks, respectively. However, the rapid acquisition method
introduced in this document does not depend on a specific version of
either of these group management protocols. In the remainder of this
document, SFGMP will refer to any group management protocol that has
Join and Leave functionalities.
Feedback Target (FT): Unicast RTCP feedback target as defined in
[RFC5760]. FT_Ap denotes a specific feedback target running on a
particular address and port.
Retransmission (or Burst) Packet: An RTP packet that is formatted as
defined in Section 4 of [RFC4588]. The payload of a retransmission
or burst packet comprises the retransmission payload header followed
by the payload of the original RTP packet.
Reference Information: The set of certain media content and metadata
information that is sufficient for an RTP receiver to start usefully
consuming a media stream. The meaning, format, and size of this
information are specific to the application and are out of the scope
of this document.
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Preamble Information: A more compact form of the whole or a subset
of the Reference Information transmitted out-of-band.
(Unicast) Burst (or Retransmission) RTP Session: The unicast RTP
session used to send one or more unicast burst RTP streams and their
associated RTCP messages. The terms "burst RTP session" and
"retransmission RTP session" can be used interchangeably.
(Unicast) Burst (Stream): A unicast stream of RTP retransmission
packets that enable an RTP receiver to rapidly acquire the Reference
Information associated with a primary multicast stream. Each burst
stream is identified by its Synchronization Source (SSRC) identifier
that is unique in the primary multicast RTP session. Following the
session-multiplexing guidelines in [RFC4588], each unicast burst
stream will use the same SSRC and Canonical Name (CNAME) as its
primary multicast RTP stream.
Retransmission Server (RS): The RTP/RTCP endpoint that can generate
the retransmission packets and the burst streams. The RS may also
generate other non-retransmission packets to aid rapid acquisition.
4. Elements of Delay in Multicast Applications
In a source-specific multicast (SSM) delivery system, there are three
major elements that contribute to the acquisition delay when an RTP
receiver switches from one multicast session to another one. These
are:
o Multicast-switching delay
o Reference Information latency
o Buffering delays
Multicast-switching delay is the delay that is experienced when
leaving the current multicast session (if any) and joining the new
multicast session. In typical systems, the multicast join and leave
operations are handled by a group management protocol. For example,
the receivers and routers participating in a multicast session can
use the Internet Group Management Protocol (IGMP) version 3 [RFC3376]
or the Multicast Listener Discovery Protocol (MLD) version 2
[RFC3810]. In either of these protocols, when a receiver wants to
join a multicast session, it sends a message to its upstream router
and the routing infrastructure sets up the multicast forwarding state
to deliver the packets of the multicast session to the new receiver.
The join times vary depending on the proximity of the upstream
router, the current state of the multicast tree, the load on the
system, and the protocol implementation. Current systems provide
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join latencies, usually less than 200 milliseconds (ms). If the
receiver had been participating in another multicast session before
joining the new session, it needs to send a Leave message to its
upstream router to leave the session. In common multicast routing
protocols, the leave times are usually smaller than the join times;
however, it is possible that the Leave and Join messages might get
lost, in which case the multicast-switching delay inevitably
increases.
Reference Information latency is the time it takes the receiver to
acquire the Reference Information. It is highly dependent on the
proximity of the actual time the receiver joined the session to the
next time the Reference Information will be sent to the receivers in
the session, whether or not the Reference Information is sent
contiguously, and the size of the Reference Information. For some
multicast flows, there is a little or no interdependency in the data,
in which case the Reference Information latency will be nil or
negligible. For other multicast flows, there is a high degree of
interdependency. One example of interest is the multicast flows that
carry compressed audio/video. For these flows, the Reference
Information latency can become quite large and be a major contributor
to the overall delay.
The buffering component of the overall acquisition delay is driven by
the way the application layer processes the payload. In many
multicast applications, an unreliable transport protocol such as UDP
[RFC0768] is often used to transmit the data packets, and the
reliability, if needed, is usually addressed through other means such
as Forward Error Correction (e.g., [RFC6015]) and retransmission.
These loss-repair methods require buffering at the receiver side to
function properly. In many applications, it is also often necessary
to de-jitter the incoming data packets before feeding them to the
application. The de-jittering process also increases the buffering
delays. Besides these network-related buffering delays, there are
also specific buffering needs that are required by the individual
applications. For example, standard video decoders typically require
a certain amount, sometimes up to a few seconds, of coded video data
to be available in the pre-decoding buffers prior to starting to
decode the video bitstream.
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RFC 6285 RAMS June 20115. Protocol Design Considerations and Their Effect on Resource Management for Rapid Acquisition
This section is for informational purposes and does not contain
requirements for implementations.
Rapid acquisition is an optimization of a system that is expected to
continue to work correctly and properly whether or not the
optimization is effective or even fails due to lost control and
feedback messages, congestion, or other problems. This is
fundamental to the overall design requirements surrounding the
protocol definition and to the resource management schemes to be
employed together with the protocol (e.g., Quality of Service (QoS)
machinery, server load management, etc). In particular, the system
needs to operate within a number of constraints:
o First, a rapid acquisition operation must fail gracefully. The
user experience must not be significantly worse for trying and
failing to complete rapid acquisition compared to simply joining
the multicast session.
o Second, providing the rapid acquisition optimizations must not
cause collateral damage to either the multicast session being
joined or other multicast sessions sharing resources with the
rapid acquisition operation. In particular, the rapid acquisition
operation must avoid interference with the multicast session that
might be simultaneously being received by other hosts. In
addition, it must also avoid interference with other multicast and
non-multicast sessions sharing the same network resources. These
properties are possible but are usually difficult to achieve.
One challenge is the existence of multiple bandwidth bottlenecks
between the receiver and the server(s) in the network providing the
rapid acquisition service. In commercial IPTV deployments, for
example, bottlenecks are often present in the aggregation network
connecting the IPTV servers to the network edge, the access links
(e.g., DSL, Data Over Cable Service Interface Specification
(DOCSIS)), and the home network of the subscribers. Some of these
links might serve only a single subscriber, limiting congestion
impact to the traffic of only that subscriber, but others can be
shared links carrying multicast sessions of many subscribers. Also
note that the state of these links can vary over time. The receiver
might have knowledge of a portion of this network or might have
partial knowledge of the entire network. The methods employed by the
devices to acquire this network state information is out of the scope
of this document. The receiver should be able to signal the server
with the bandwidth that it believes it can handle. The server also
needs to be able to rate limit the flow in order to stay within the
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performance envelope that it knows about. Both the server and
receiver need to be able to inform the other of changes in the
requested and delivered rates. However, the protocol must be robust
in the presence of packet loss, so this signaling must include the
appropriate default behaviors.
A second challenge is that for some uses (e.g., high-bitrate video)
the unicast burst bitrate is high while the flow duration of the
unicast burst is short. This is because the purpose of the unicast
burst is to allow the RTP receiver to join the multicast quickly and
thereby limit the overall resources consumed by the burst. Such
high-bitrate, short-duration flows are not amenable to conventional
admission-control techniques. For example, end-to-end per-flow
signaled admission-control techniques such as Resource Reservation
Protocol (RSVP) have too much latency and control channel overhead to
be a good fit for rapid acquisition. Similarly, using a TCP (or TCP-
like) approach with a 3-way handshake and slow-start to avoid
inducing congestion would defeat the purpose of attempting rapid
acquisition in the first place by introducing many round-trip times
(RTTs) of delay.
These observations lead to certain unavoidable requirements and goals
for a rapid acquisition protocol. These are:
o The protocol must be designed to allow a deterministic upper bound
on the extra bandwidth used (compared to just joining the
multicast session). A reasonable size bound is e*B, where B is
the nominal bandwidth of the primary multicast streams and e is an
excess-bandwidth coefficient. The total duration of the unicast
burst must have a reasonable bound; long unicast bursts devolve to
the bandwidth profile of multi-unicast for the whole system.
o The scheme should minimize (or better eliminate) the overlap of
the unicast burst and the primary multicast stream. This
minimizes the window during which congestion could be induced on a
bottleneck link compared to just carrying the multicast or unicast
packets alone.
o The scheme must minimize (or better eliminate) any gap between the
unicast burst and the primary multicast stream, which has to be
repaired later or, in the absence of repair, will result in loss
being experienced by the application.
In addition to the above, there are some other protocol design issues
to be considered. First, there is at least one RTT of "slop" in the
control loop. In starting a rapid acquisition burst, this manifests
as the time between the client requesting the unicast burst and the
burst description and/or the first unicast burst packets arriving at
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RFC 6285 RAMS June 2011
the receiver. For managing and terminating the unicast burst, there
are two possible approaches for the control loop. First, the
receiver can adapt to the unicast burst as received, converge based
on observation, and explicitly terminate the unicast burst with a
second control loop exchange (which takes a minimum of one RTT, just
as starting the unicast burst does). Alternatively, the server
generating the unicast burst can precompute the burst parameters
based on the information in the initial request and tell the receiver
the burst duration.
The protocol described in the next section allows either method of
controlling the rapid acquisition unicast burst.
6. Rapid Acquisition of Multicast RTP Sessions (RAMS)
We start this section with an overview of the Rapid Acquisition of
Multicast RTP Sessions (RAMS) method.
6.1. Overview
[RFC5760] specifies an extension to the RTP Control Protocol (RTCP)
to use unicast feedback in an SSM session. It defines an
architecture that introduces the concept of Distribution Source,
which, in an SSM context, distributes the RTP data and redistributes
RTCP information to all RTP receivers. This RTCP information is
retrieved from the Feedback Target, to which RTCP unicast feedback
traffic is sent. One or more entities different from the
Distribution Source MAY implement the feedback target functionality,
and different RTP receivers MAY use different feedback targets.
This document builds further on these concepts to reduce the
acquisition delay when an RTP receiver joins a multicast session at a
random point in time by introducing the concept of the Burst Source
and new RTCP feedback messages. The Burst Source has a cache where
the most recent packets from the primary multicast RTP session are
continuously stored. When an RTP receiver wants to receive a primary
multicast stream, it can first request a unicast burst from the Burst
Source before it joins the SSM session. In this burst, the packets
are formatted as RTP retransmission packets [RFC4588] and carry
Reference Information. This information allows the RTP receiver to
start usefully consuming the RTP packets sent in the primary
multicast RTP session.
Using an accelerated bitrate (as compared to the nominal bitrate of
the primary multicast stream) for the unicast burst implies that at a
certain point in time, the payload transmitted in the unicast burst
is going to be the same as the payload in the associated multicast
stream, i.e., the unicast burst will catch up with the primary
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RFC 6285 RAMS June 2011
multicast stream. At this point, the RTP receiver no longer needs to
receive the unicast burst and can join the SSM session. This method
is referred to as the Rapid Acquisition of Multicast RTP Sessions
(RAMS).
This document defines extensions to [RFC4585] for an RTP receiver to
request a unicast burst as well as for additional control messaging
that can be leveraged during the acquisition process.
6.2. Message Flows
As shown in Figure 2, the main entities involved in rapid acquisition
and the message flows are:
o Multicast Source
o Feedback Target (FT)
o Burst/Retransmission Source (BRS)
o RTP Receiver (RTP_Rx)
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The FT is involved in the primary multicast RTP session and receives
unicast feedback for that session as in [RFC5760]. The BRS is
involved in the unicast burst (or retransmission) RTP session and
transmits the unicast burst and retransmission packets formatted as
RTP retransmission packets [RFC4588] in a single separate unicast RTP
session to each RTP_Rx. In the unicast burst RTP session, as in any
other RTP session, the BRS and RTP_Rx regularly send the periodic
sender and receiver reports, respectively.
The unicast burst is started by an RTCP feedback message that is
defined in this document based on the common packet format provided
in [RFC4585]. An RTP retransmission is triggered by an RTCP NACK
message defined in [RFC4585]. Both of these messages are sent to the
FT of the primary multicast RTP session and can start the unicast
burst/retransmission RTP session.
In the extended RTP profile for RTCP-based feedback (RTP/Audio-Visual
Profile with Feedback (AVPF)), there are certain rules that apply to
scheduling of both of these messages sent to the FT in the primary
multicast RTP session; these are detailed in Section 3.5 of
[RFC4585]. One of the rules states that in a multi-party session
such as the SSM sessions we are considering in this specification, an
RTP_Rx cannot send an RTCP feedback message for a minimum of one
second after joining the session (i.e., Tmin=1.0 second). While this
rule has the goal of avoiding problems associated with flash crowds
in typical multi-party sessions, it defeats the purpose of rapid
acquisition. Furthermore, when RTP receivers delay their messages
requesting a burst by a deterministic or even a random amount, it
still does not make a difference since such messages are not related
and their timings are independent from each other. Thus, in this
specification, we initialize Tmin to zero and allow the RTP receivers
to send a burst request message right at the beginning. For the
subsequent messages (e.g., updated requests) during rapid
acquisition, the timing rules of [RFC4585] still apply.
Figure 3 depicts an example of messaging flow for rapid acquisition.
The RTCP feedback messages are explained below. The optional
messages are indicated in parentheses, and they might or might not be
present during rapid acquisition. In a scenario where rapid
acquisition is performed by a feedback target co-located with the
media sender, the same method (with the identical message flows)
still applies.
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RFC 6285 RAMS June 2011
-------> Multicast RTP Flow
.-.-.-.> Multicast RTCP Flow
.=.=.=.> Unicast RTCP Reports
~~~~~~~> Unicast RTCP Feedback Messages
=======> SFGMP Messages
.......> Unicast RTP Flow
Figure 3: Message Flows for Unicast-Based Rapid Acquisition
This document defines the expected behaviors of the RS and RTP_Rx.
It is instructive to consider independently operating implementations
on the RS and RTP_Rx that request the burst, describe the burst,
start the burst, join the multicast session, and stop the burst.
These implementations send messages to each other, but provisions are
needed for the cases where the control messages get lost, or
reordered, or are not being delivered to their destinations.
The following steps describe rapid acquisition in detail:
1. Port Mapping Setup: For the primary multicast RTP session, the
RTP and RTCP destination ports are declaratively specified
(refer to Section 8 for examples in SDP). However, the RTP_Rx
needs to choose its RTP and RTCP receive ports for the unicast
burst RTP session. Since this unicast session is established
after the RTP_Rx has sent a RAMS Request (RAMS-R) message as
unicast feedback in the primary multicast RTP session, the
RTP_Rx MUST first set up the port mappings between the unicast
and multicast sessions and send this mapping information to the
FT along with the RAMS-R message so that the BRS knows how to
communicate with the RTP_Rx.
The details of this setup procedure are explained in [RFC6284].
Other NAT-related issues are left to Section 9 to keep the
present discussion focused on the RAMS message flows.
2. Request: The RTP_Rx sends a rapid acquisition request (RAMS-R)
for the primary multicast RTP session to the unicast feedback
target of that session. The request contains the SSRC
identifier of the RTP_Rx (aka SSRC of packet sender) and can
contain the media sender SSRC identifier(s) of the primary
multicast stream(s) of interest (aka SSRC of media source). The
RAMS-R message can contain parameters that constrain the burst,
such as the buffer and bandwidth limits.
Before joining the SSM session, the RTP_Rx learns the addresses
for the multicast source, group, and RS by out-of-band means.
If the RTP_Rx desires to rapidly acquire only a subset of the
primary multicast streams available in the primary multicast RTP
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RFC 6285 RAMS June 2011
session, the RTP_Rx MUST also acquire the SSRC identifiers for
the desired RTP streams out-of-band. Based on this information,
the RTP_Rx populates the desired SSRC(s) in the RAMS-R message.
When the FT successfully receives the RAMS-R message, the BRS
responds to it by accepting or rejecting the request.
Immediately before the BRS sends any RTP or RTCP packet(s)
described below, it establishes the unicast burst RTP session.
3. Response: The BRS sends RAMS Information (RAMS-I) message(s) to
the RTP_Rx to convey the status for the burst(s) requested by
the RTP_Rx.
If the primary multicast RTP session associated with the FT_Ap
(a specific feedback target running on a particular address and
port) on which the RAMS-R message was received contains only a
single primary multicast stream, the BRS SHALL always use the
SSRC of the RTP stream associated with the FT_Ap in the RAMS-I
message(s) regardless of the media sender SSRC requested in the
RAMS-R message. In such cases the 'ssrc' attribute MAY be
omitted from the media description. If the requested SSRC and
the actual media sender SSRC do not match, the BRS MUST
explicitly populate the correct media sender SSRC in the initial
RAMS-I message (see Section 7.3).
The FT_Ap could also be associated with an RTP session that
carries two or more primary multicast streams. If the RTP_Rx
wants to issue a collective request to receive the whole primary
multicast RTP session, it does not need the 'ssrc' attributes to
be described in the media description.
If the FT_Ap is associated with two or more RTP sessions,
RTP_Rx's request will be ambiguous. To avoid any ambiguity,
each FT_Ap MUST be only associated with a single RTP session.
If the RTP_Rx is willing to rapidly acquire only a subset of the
primary multicast streams, the RTP_Rx MUST list all the media
sender SSRC(s) denoting the stream(s) it wishes to acquire in
the RAMS-R message. Upon receiving such a message, the BRS MAY
accept the request for all or a subset of the media sender
SSRC(s) that match the RTP stream(s) it serves. The BRS MUST
reject all other requests with an appropriate response code.
* Reject Responses: The BRS MUST take into account any
limitations that may have been specified by the RTP_Rx in the
RAMS-R message when making a decision regarding the request.
If the RTP_Rx has requested to acquire the whole primary
multicast RTP session but the BRS cannot provide a rapid
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RFC 6285 RAMS June 2011
acquisition service for any of the primary multicast streams,
the BRS MUST reject the request via a single RAMS-I message
with a collective reject response code, which is defined as
510 in Section 11.6 and whose media sender SSRC field is set
to one of SSRCs served by this FT_Ap. Upon receiving this
RAMS-I message, the RTP_Rx abandons the rapid acquisition
attempt and can immediately join the multicast session by
sending an SFGMP Join message towards its upstream multicast
router.
In all other cases, the BRS MUST send a separate RAMS-I
message with the appropriate 5xx-level response code (as
defined in Section 11.6) for each primary multicast stream
that has been requested by the RTP_Rx but cannot be served by
the BRS. There could be multiple reasons why the BRS has
rejected the request; however, the BRS chooses the most
appropriate response code to inform the RTP_Rx.
Upon receiving a reject response indicating a transient
problem such as insufficient BRS or network resources, if the
RTP_Rx wants to retry sending the same request, the RTP_Rx
MUST follow the RTCP timer rules of [RFC4585] to allow the
transient problems to go away. However, if the reject
response indicates a non-transient problem (such as the ones
reported by response codes 504, 505, and 506), the RTP_Rx
MUST NOT attempt a retry. Different response codes have
different scopes. Refer to Section 7.3.1 for details.
The BRS can employ a policing mechanism against the broken
RTP_Rx implementations that are not following these rules.
See Section 10 for details.
* Accept Responses: The BRS MUST send at least one separate
RAMS-I message with the appropriate response code (either
zero indicating a private response or response code 200
indicating success as listed in Section 11.6) for each
primary multicast stream that has been requested by the
RTP_Rx and will be served by the BRS. Such RAMS-I messages
comprise fields that can be used to describe the individual
unicast burst streams. When there is a RAMS-R request for
multiple primary multicast streams, the BRS MUST include all
the individual RAMS-I messages corresponding to that RAMS-R
request in the same compound RTCP packet if these messages
fit in the same packet. Note that if the BRS is sending only
the preamble information but not the whole unicast burst
stream, it will not accept the request but will send a
response code 511 instead, as defined in Section 11.6.
Ver Steeg, et al. Standards Track [Page 19]

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The RAMS-I message carries the RTP sequence number of the
first packet transmitted in the respective RTP stream to
allow the RTP_Rx to detect any missing initial packet(s).
When the BRS accepts a request for a primary multicast
stream, this field MUST always be populated in the RAMS-I
message(s) sent for this particular primary multicast stream.
It is RECOMMENDED that the BRS sends a RAMS-I message at the
start of the burst so that the RTP_Rx can quickly detect any
missing initial packet(s).
It is possible that the RAMS-I message for a primary multicast
stream can get delayed or lost, and the RTP_Rx can start
receiving RTP packets before receiving a RAMS-I message. An
RTP_Rx implementation MUST NOT assume it will quickly receive
the initial RAMS-I message. For redundancy purposes, it is
RECOMMENDED that the BRS repeats the RAMS-I messages multiple
times as long as it follows the RTCP timer rules defined in
[RFC4585].
4. Unicast Burst: For the primary multicast stream(s) for which
the request is accepted, the BRS starts sending the unicast
burst(s) that comprises one or more RTP retransmission packets
sent in the unicast burst RTP session. In some applications,
the BRS can send preamble information data to the RTP_Rx in
addition to the requested burst to prime the media decoder(s).
However, for the BRS to send the preamble information in a
particular format, explicit signaling from the RTP_Rx is
required. In other words, the BRS MUST NOT send preamble
information in a particular format unless the RTP_Rx has
signaled support for that format in the RAMS-R message through a
public or private extension as defined in Section 7.1.
The format of this preamble data is RTP-payload specific and not
specified here.
5. Updated Request: The RTP_Rx MAY send an updated RAMS-R message
(as unicast feedback in the primary multicast RTP session) with
a different value for one or more fields of an earlier RAMS-R
message. The BRS MUST be able to detect whether a burst is
already planned for or being transmitted to this particular
RTP_Rx for this particular media sender SSRC. If there is an
existing burst planned for or being transmitted, the newly
arriving RAMS-R is an updated request; otherwise, it is a new
request. It is also possible that the RTP_Rx has sent an
improperly formatted RAMS-R message or used an invalid value in
the RAMS-R message. If notified by the BRS using a 4xx-level
Ver Steeg, et al. Standards Track [Page 20]

RFC 6285 RAMS June 2011
response code (as defined in Section 11.6) and only after
following the timing rules of [RFC4585], the RTP_Rx MAY resend
its corrected request.
The BRS determines the identity of the requesting RTP_Rx by
looking at its canonical name identifier (CNAME item in the
source description (SDES)). Thus, the RTP_Rx MUST choose a
method that ensures it uses a unique CNAME identifier. Such
methods are provided in [RFC6222]. In addition to one or more
fields with updated values, an updated RAMS-R message may also
include the fields whose values did not change.
Upon receiving an updated request, the BRS can use the updated
values for sending/shaping the burst or refine the values and
use the refined values for sending/shaping the burst.
Subsequently, the BRS MAY send an updated RAMS-I message in the
unicast burst RTP session to indicate the changes it made.
It is an implementation-dependent decision for an RTP_RX whether
and when to send an updated request. However, note that the
updated request messages can get delayed and arrive at the BRS
after the initial unicast burst was completed. In this case,
the updated request message can trigger a new unicast burst, and
by then if the RTP_Rx has already started receiving multicast
data, a congestion is likely to occur. In this case, the RTP_Rx
can detect this only after a delay, and then it can try to
terminate the new burst. However, in the meantime, the RTP_Rx
can experience packet loss or other problems. This and other
similar corner cases SHOULD be carefully examined if updated
requests are to be used.
6. Updated Response: The BRS can send more than one RAMS-I message
in the unicast burst RTP session, e.g., to update the value of
one or more fields in an earlier RAMS-I message. The updated
RAMS-I messages might or might not be a direct response to a
RAMS-R message. The BRS can also send updated RAMS-I messages
to signal the RTP_Rx in real time to join the SSM session when
the BRS had already sent an initial RAMS-I message, e.g., at the
start of the burst. The RTP_Rx depends on the BRS to learn the
join time, which can be conveyed by the first RAMS-I message or
can be sent/revised in a later RAMS-I message. If the BRS is
not capable of determining the join time in the initial RAMS-I
message, the BRS MUST send another RAMS-I message (with the join
time information) later.
7. Multicast Join Signaling: The RAMS-I message allows the BRS to
signal explicitly when the RTP_Rx needs to send the SFGMP Join
message. The RTP_Rx SHOULD use this information from the most
Ver Steeg, et al. Standards Track [Page 21]

RFC 6285 RAMS June 2011
recent RAMS-I message unless it has more accurate information.
If the request is accepted, this information MUST be conveyed in
at least one RAMS-I message, and its value MAY be updated by
subsequent RAMS-I messages.
There can be missing packets if the RTP_Rx joins the multicast
session too early or too late. For example, if the RTP_Rx
starts receiving the primary multicast stream while it is still
receiving the unicast burst at a high excess bitrate, this can
result in an increased risk of packet loss. Or, if the RTP_Rx
joins the multicast session some time after the unicast burst is
finished, there can be a gap between the burst and multicast
data (a number of RTP packets might be missing). In both cases,
the RTP_Rx can issue retransmission requests (via RTCP NACK
messages sent as unicast feedback in the primary multicast RTP
session) [RFC4585] to the FT entity of the RS to fill the gap.
The BRS might or might not respond to such requests. When it
responds and the response causes significant changes in one or
more values reported earlier to the RTP_Rx, an updated RAMS-I
SHOULD be sent to the RTP_Rx.
8. Multicast Receive: After the join, the RTP_Rx starts receiving
the primary multicast stream(s).
9. Terminate: The BRS can know when it needs to ultimately stop
the unicast burst based on its parameters. However, the RTP_Rx
may need to ask the BRS to terminate the burst prematurely or at
a specific sequence number. For this purpose, the RTP_Rx uses
the RAMS Termination (RAMS-T) message sent as RTCP feedback in
the unicast burst RTP session. A separate RAMS-T message is
sent for each primary multicast stream served by the BRS unless
an RTCP BYE message has been sent in the unicast burst RTP
session as described in Step 10. For the burst requests that
were rejected by the BRS, there is no need to send a RAMS-T
message.
If the RTP_Rx wants to terminate a burst prematurely, it MUST
send a RAMS-T message for the SSRC of the primary multicast
stream it wishes to terminate. This message is sent in the
unicast burst RTP session. Upon receiving this message, the BRS
MUST terminate the unicast burst. If the RTP_Rx requested to
acquire the entire primary multicast RTP session but wants to
terminate this request before it learns the individual media
sender SSRC(s) via RAMS-I message(s) or RTP packets, the RTP_Rx
cannot use RAMS-T message(s) and thus MUST send an RTCP BYE
message in the unicast burst RTP session to terminate the
request.
Ver Steeg, et al. Standards Track [Page 22]

RFC 6285 RAMS June 2011
Otherwise, the default behavior for the RTP_Rx is to send a
RAMS-T message in the unicast burst RTP session immediately
after it joins the multicast session and has started receiving
multicast packets. In that case, the RTP_Rx MUST send a RAMS-T
message with the sequence number of the first RTP packet
received in the primary multicast stream. Then, the BRS MUST
terminate the respective burst after it sends the unicast burst
packet whose Original Sequence Number (OSN) field in the RTP
retransmission payload header matches this number minus one. If
the BRS has already sent that unicast burst packet when the
RAMS-T message arrives, the BRS MUST terminate the respective
burst immediately.
If an RTCP BYE message has not been issued yet as described in
Step 10, the RTP_Rx MUST send at least one RAMS-T message for
each primary multicast stream served by the BRS. The RAMS-T
message(s) MUST be sent to the BRS in the unicast burst RTP
session. Against the possibility of a message loss, it is
RECOMMENDED that the RTP_Rx repeats the RAMS-T messages multiple
times as long as it follows the RTCP timer rules defined in
[RFC4585].
The binding between RAMS-T and ongoing bursts is achieved
through RTP_Rx's CNAME identifier and packet sender and media
sender SSRCs. Choosing a globally unique CNAME makes sure that
the RAMS-T messages are processed correctly.
10. Terminate with RTCP BYE: When the RTP_Rx is receiving one or
more burst streams, if the RTP_Rx becomes no longer interested
in acquiring any of the primary multicast streams, the RTP_Rx
SHALL issue an RTCP BYE message for the unicast burst RTP
session and another RTCP BYE message for the primary multicast
RTP session. These RTCP BYE messages are sent to the BRS and FT
logical entities, respectively.
Upon receiving an RTCP BYE message, the BRS MUST terminate the
rapid acquisition operation and cease transmitting any further
burst packets and retransmission packets. If support for
[RFC5506] has been signaled, the RTCP BYE message MAY be sent in
a reduced-size RTCP packet. Otherwise, Section 6.1 of [RFC3550]
mandates the RTCP BYE message always be sent with a sender or
receiver report in a compound RTCP packet. If no data has been
received, an empty receiver report MUST be still included. With
the information contained in the receiver report, the RS can
figure out how many duplicate RTP packets have been delivered to
the RTP_Rx (note that this will be an upper-bound estimate as
one or more packets might have been lost during the burst
Ver Steeg, et al. Standards Track [Page 23]

RFC 6285 RAMS June 2011
transmission). The impact of duplicate packets and measures
that can be taken to minimize the impact of receiving duplicate
packets will be addressed in Section 6.4.
Since an RTCP BYE message issued for the unicast burst RTP
session terminates that session and ceases transmitting any
further packets in that session, there is no need for sending
explicit RAMS-T messages, which would only terminate their
respective bursts.
For the purpose of gathering detailed information about RTP_Rx's
rapid acquisition experience, [MULTICAST-ACQ] defines an RTCP
Extended Report (XR) Block. This report is designed to be payload-
independent; thus, it can be used by any multicast application that
supports rapid acquisition.
6.3. Synchronization of Primary Multicast Streams
When an RTP_RX acquires multiple primary multicast streams, it might
need to synchronize them for playout. This synchronization is
achieved by the help of the RTCP sender reports [RFC3550]. If the
playout will start before the RTP_Rx has joined the multicast
session, the RTP_Rx needs to receive the information reflecting the
synchronization among the primary multicast streams early enough so
that it can play out the media in a synchronized fashion.
The suggested approach is to use the RTP header extension mechanism
[RFC5285] and convey the synchronization information in a header
extension as defined in [RFC6051]. Per [RFC4588], "if the original
RTP header carried an RTP header extension, the retransmission packet
SHOULD carry the same header extension". Thus, as long as the
multicast source emits a header extension with the synchronization
information frequently enough, there is no additional task that needs
to be carried out by the BRS. The synchronization information will
be sent to the RTP_Rx along with the burst packets. The frequent
header extensions sent in the primary multicast RTP sessions also
allow rapid synchronization of the RTP streams for the RTP receivers
that do not support RAMS or that directly join the multicast session
without running RAMS. Thus, in RAMS applications, it is RECOMMENDED
that the multicast sources frequently send synchronization
information by using header extensions following the rules presented
in [RFC6051]. The regular sender reports are still sent in the
unicast session by following the rules of [RFC3550].
Ver Steeg, et al. Standards Track [Page 24]

RFC 6285 RAMS June 20116.4. Burst Shaping and Congestion Control in RAMS
This section provides informative guidelines about how the BRS can
shape the transmission of the unicast burst and how congestion can be
dealt with in the RAMS process. When the RTP_Rx detects that the
unicast burst is causing severe congestion, it can prefer to send a
RAMS-T message immediately to stop the unicast burst.
A higher bitrate for the unicast burst naturally conveys the
Reference Information and media content to the RTP_Rx faster. This
way, the RTP_Rx can start consuming the data sooner, which results in
a faster acquisition. A higher bitrate also represents a better
utilization of the BRS resources. As the burst may continue until it
catches up with the primary multicast stream, the higher the bursting
bitrate, the less data the BRS needs to transmit. However, a higher
bitrate for the burst also increases the chances for congestion-
caused packet loss. Thus, as discussed in Section 5, there has to be
an upper bound on the bandwidth used by the burst.
When the BRS transmits the unicast burst, it is expected to take into
account all available information to prevent any packet loss that
might take place during the bursting as a result of buffer overflow
on the path between the RS and RTP_Rx and at the RTP_Rx itself. The
bursting bitrate can be determined by taking into account the
following information, when available:
(a) Information obtained via the RAMS-R message, such as Max RAMS
Buffer Fill Requirement and/or Max Receive Bitrate (see
Section 7.2).
(b) Information obtained via RTCP receiver reports provided by the
RTP_Rx in the retransmission session, allowing in-session
bitrate adaptations for the burst. When these receiver reports
indicate packet loss, this can indicate a certain congestion
state in the path from the RS to the RTP_Rx.
(c) Information obtained via RTCP NACKs provided by the RTP_Rx in
the primary multicast RTP session, allowing in-session bitrate
adaptations for the burst. Such RTCP NACKs are transmitted by
the RTP_Rx in response to packet loss detection in the burst.
NACKs can indicate a certain congestion state on the path from
the RS to RTP_Rx.
(d) There can be other feedback received from the RTP_Rx, e.g., in
the form of Explicit Congestion Notification - Congestion
Experienced (ECN-CE) markings [ECN-FOR-RTP] that can influence
in-session bitrate adaptation.
Ver Steeg, et al. Standards Track [Page 25]

RFC 6285 RAMS June 2011
(e) Information obtained via updated RAMS-R messages, allowing in-
session bitrate adaptations, if supported by the BRS.
(f) Transport protocol-specific information. For example, when
Datagram Congestion Control Protocol (DCCP) is used to transport
the RTP burst, the ACKs from the DCCP client can be leveraged by
the BRS / DCCP server for burst shaping and congestion control.
(g) Preconfigured settings for each RTP_Rx or a set of RTP_Rxs that
indicate the upper-bound bursting bitrates for which no packet
loss will occur as a result of congestion along the path of the
RS to RTP_Rx. For example, in managed IPTV networks, where the
bottleneck bandwidth along the end-to-end path is known and
where the network between the RS and this link is provisioned
and dimensioned to carry the burst streams, the bursting bitrate
does not exceed the provisioned value. These settings can also
be dynamically adapted using application-aware knowledge.
The BRS chooses the initial burst bitrate as follows:
o When using RAMS in environments as described in (g), the BRS MUST
transmit the burst packets at an initial bitrate higher than the
nominal bitrate but within the engineered or reserved bandwidth
limit.
o When the BRS cannot determine a reliable bitrate value for the
unicast burst (using (a) through (g)), it is desirable for the BRS
to choose an appropriate initial bitrate not above the nominal
bitrate and increase it gradually unless a congestion is detected.
In both cases, during the burst transmission, the BRS MUST
continuously monitor for packet losses as a result of congestion by
means of one or more of the mechanisms described in (b) through (f).
When the BRS relies on RTCP receiver reports, sufficient bandwidth
needs to be provided to RTP_Rx for RTCP transmission in the unicast
burst RTP session. To achieve a reasonable fast adaptation against
congestion, it is recommended that the RTP_Rx sends a receiver report
at least once every two RTTs between the RS and RTP_Rx. Although the
specific heuristics and algorithms that deduce a congestion state and
how the BRS acts subsequently are outside the scope of this
specification, the following two methods are the best practices:
o Upon detection of a significant amount of packet loss, which the
BRS attributes to congestion, the BRS decreases the burst bitrate.
The rate by which the BRS increases and decreases the bitrate for
the burst can be determined by a TCP-friendly bitrate adaptation
algorithm for RTP over UDP or, in the case of (f), by the
congestion control algorithms defined in DCCP [RFC5762].
Ver Steeg, et al. Standards Track [Page 26]

RFC 6285 RAMS June 2011
o If the congestion is persistent and the BRS has to reduce the
burst bitrate to a point where the RTP_Rx buffer might underrun or
the burst will consume too many resources, the BRS terminates the
burst and transmits a RAMS-I message to RTP_Rx with the
appropriate response code. It is then up to RTP_Rx to decide when
to join the multicast session.
Even though there is no congestion experienced during the burst,
congestion may anyway arise near the end of the burst as the RTP_Rx
eventually needs to join the multicast session. During this brief
period, both the burst packets and the multicast packets can be
simultaneously received by the RTP_Rx, thus enhancing the risk of
congestion.
Since the BRS signals the RTP_Rx when the BRS expects the RTP_Rx to
send the SFGMP Join message, the BRS can have a rough estimate of
when the RTP_Rx will start receiving multicast packets in the SSM
session. The BRS can keep on sending burst packets but reduces the
bitrate accordingly at the appropriate instant by taking the bitrate
of the whole SSM session into account. If the BRS ceases
transmitting the burst packets before the burst catches up, any gap
resulting from this imperfect switch-over by the RTP_Rx can be later
repaired by requesting retransmissions for the missing packets from
the RS. The retransmissions can be shaped by the BRS to make sure
that they do not cause collateral loss in the primary multicast RTP
session and the unicast burst RTP session.
6.5. Failure Cases
In this section, we examine the implications of losing the RAMS-R,
RAMS-I, or RAMS-T messages and other failure cases.
When the RTP_Rx sends a RAMS-R message to initiate a rapid
acquisition but the message gets lost and the FT does not receive it,
the RTP_Rx will get neither a RAMS-I message nor a unicast burst. In
this case, the RTP_Rx MAY resend the request when it is eligible to
do so based on the RTCP timer rules defined in [RFC4585]. Or, after
a reasonable amount of time, the RTP_Rx can time out (based on the
previous observed response times) and immediately join the SSM
session.
In the case where RTP_Rx starts receiving a unicast burst but does
not receive a corresponding RAMS-I message within a reasonable amount
of time, the RTP_Rx can either discard the burst data or decide not
to interrupt the unicast burst and be prepared to join the SSM
session at an appropriate time it determines or as indicated in a
subsequent RAMS-I message (if available). If the network is subject
Ver Steeg, et al. Standards Track [Page 27]

RFC 6285 RAMS June 2011
to packet loss, it is RECOMMENDED that the BRS repeats the RAMS-I
messages multiple times based on the RTCP timer rules defined in
[RFC4585].
In the failure cases where the RAMS-I message is lost or the RAMS-R
message is lost and the RTP_Rx gives up, the RTP_Rx MUST still
terminate the burst(s) it requested by following the rules described
in Section 6.2.
In the case where a RAMS-T message sent by the RTP_Rx does not reach
its destination, the BRS can continue sending burst packets even
though the RTP_Rx no longer needs them. If an RTP_Rx is receiving
burst packets it no longer needs after sending a RAMS-T message, it
is RECOMMENDED that the RTP_Rx repeats the RAMS-T message multiple
times based on the RTCP timer rules defined in [RFC4585]. The BRS
MUST be provisioned to terminate the burst when it can no longer send
the burst packets faster than it receives the primary multicast
stream packets.
Section 6.3.5 of [RFC3550] explains the rules pertaining to timing
out an SSRC. When the BRS accepts to serve the requested burst(s)
and establishes the retransmission session, the BRS needs to check
the liveness of the RTP_Rx via the RTCP messages and reports the
RTP_Rx sends. The default rules explained in [RFC3550] apply in RAMS
as well.
7. Encoding of the Signaling Protocol in RTCP
This section defines the formats of the RTCP transport-layer feedback
messages that are exchanged between the retransmission server (RS)
and RTP receiver (RTP_Rx) during rapid acquisition. These messages
are referred to as the RAMS Messages. They are payload-independent
and MUST be used by all RTP-based multicast applications that support
rapid acquisition regardless of the payload they carry.
Payload-specific feedback messages are not defined in this document.
However, further optional payload-independent and payload-specific
information can be included in the exchange.
The common packet format for the RTCP feedback messages is defined in
Section 6.1 of [RFC4585] and is also sketched in Figure 4.
Ver Steeg, et al. Standards Track [Page 28]

RFC 6285 RAMS June 2011
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P| FMT | PT | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of packet sender |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of media source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Feedback Control Information (FCI) :
: :
Figure 4: The Common Packet Format for the RTCP Feedback Messages
Each feedback message has a fixed-length field for version, padding,
feedback message type (FMT), packet type (PT), length, SSRC of packet
sender, SSRC of media source, and a variable-length field for
feedback control information (FCI).
In the RAMS messages, the PT field is set to RTPFB (205) and the FMT
field is set to RAMS (6). Individual RAMS messages are identified by
a sub-field called sub-feedback message type (SFMT). Any Reserved
field SHALL be set to zero and ignored.
Depending on the specific scenario and timeliness/importance of a
RAMS message, it can be desirable to send it in a reduced-size RTCP
packet [RFC5506]. However, unless support for [RFC5506] has been
signaled, compound RTCP packets MUST be used by following [RFC3550]
rules.
Following the rules specified in [RFC3550], all integer fields in the
messages defined below are carried in network-byte order, that is,
most significant byte (octet) first, also known as big-endian.
Unless otherwise stated, numeric constants are in decimal (base 10).
7.1. Extensions
To improve the functionality of the RAMS method in certain
applications, it can be desirable to define new fields in the RAMS
Request, Information, and Termination messages. Such fields MUST be
encoded as Type-Length-Value (TLV) elements as described below and
sketched in Figure 5:
o Type: A single-octet identifier that defines the type of the
parameter represented in this TLV element.
Ver Steeg, et al. Standards Track [Page 29]

RFC 6285 RAMS June 2011
o Length: A two-octet field that indicates the length (in octets)
of the TLV element excluding the Type and Length fields and the
8-bit Reserved field between them. This length does not include
any padding that is required for alignment.
o Value: Variable-size set of octets that contains the specific
value for the parameter.
In the extensions, the Reserved field SHALL be set to zero and
ignored. If a TLV element does not fall on a 32-bit boundary, the
last word MUST be padded to the boundary using further bits set to
zero.
An RTP_Rx or BRS MAY include vendor-neutral and private extensions in
any RAMS message. The RTP_Rx or BRS MUST place such extensions after
the mandatory fields and mandatory TLV elements (if any) and MAY
place such extensions in any order. The RTP_Rx or BRS MUST NOT
include multiple TLV elements with the same Type value in a RAMS
message.
The support for extensions (unless specified otherwise) is OPTIONAL.
An RTP_Rx or BRS receiving a vendor-neutral or private extension that
it does not understand MUST ignore that extension.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Reserved | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Value :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Structure of a TLV Element
7.1.1. Vendor-Neutral Extensions
If the goal in defining new TLV elements is to extend the
functionality in a vendor-neutral manner, they MUST be registered
with IANA through the guidelines provided in Section 11.5.
The current document defines several vendor-neutral extensions in the
subsequent sections.
7.1.2. Private Extensions
It is desirable to allow vendors to use private extensions in a TLV
format. For interoperability, such extensions must not collide with
each other.
Ver Steeg, et al. Standards Track [Page 30]

RFC 6285 RAMS June 2011
A certain range of TLV Types (between and including 128 and 254 ) is
reserved for private extensions (refer to Section 11.5). IANA
management for these extensions is unnecessary, and they are the
responsibility of individual vendors.
The structure that MUST be used for the private extensions is
depicted in Figure 6. Here, the enterprise numbers are as listed on
http://www.iana.org. This will ensure the uniqueness of the private
extensions and avoid any collision.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Reserved | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Enterprise Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Value :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: Structure of a Private Extension
7.2. RAMS Request
The RAMS Request (RAMS-R) message is identified by SFMT=1. This
message is sent as unicast feedback in the primary multicast RTP
session by the RTP_Rx to request rapid acquisition of a primary
multicast RTP session or one or more primary multicast streams
belonging to the same primary multicast RTP session. In the RAMS-R
message, the RTP_Rx MUST set both the packet sender SSRC and media
sender SSRC fields to its own SSRC since the media sender SSRC may
not be known. The RTP_Rx MUST provide explicit signaling as
described below to request stream(s). This minimizes the chances of
accidentally requesting a wrong primary multicast stream.
The FCI field MUST contain only one RAMS Request. The FCI field has
the structure depicted in Figure 7.
The semantics of the RAMS-R message is independent of the payload
type carried in the primary multicast RTP session.
Ver Steeg, et al. Standards Track [Page 31]

RFC 6285 RAMS June 2011
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SFMT=1 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Requested Media Sender SSRC(s) :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Optional TLV-encoded Fields (and Padding, if needed) :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: FCI Field Syntax for the RAMS Request Message
o Requested Media Sender SSRC(s): Mandatory TLV element that lists
the media sender SSRC(s) requested by the RTP_Rx. The BRS MUST
ignore the media sender SSRC specified in the header of the RAMS-R
message.
If the Length field is set to zero (i.e., no media sender SSRC is
listed), it means that the RTP_Rx is requesting to rapidly acquire
the entire primary multicast RTP session. Otherwise, the RTP_Rx
lists the individual media sender SSRCs in this TLV element and
sets the Length field of the TLV element to 4*n, where n is the
number of SSRC entries.
Type: 1
o Min RAMS Buffer Fill Requirement (32 bits): Optional TLV element
that denotes the minimum milliseconds of data that the RTP_Rx
desires to have in its buffer before allowing the data to be
consumed by the application.
The RTP_Rx can have knowledge of its buffering requirements.
These requirements can be application and/or device specific. For
instance, the RTP_Rx might need to have a certain amount of data
in its application buffer to handle transmission jitter and/or to
be able to support error-control methods. If the BRS is told the
minimum buffering requirement of the receiver, the BRS can tailor
the burst(s) more precisely, e.g., by choosing an appropriate
starting point. The methods used by the RTP_Rx to determine this
value are application specific and thus out of the scope of this
document.
Ver Steeg, et al. Standards Track [Page 32]

RFC 6285 RAMS June 2011
If specified, the amount of backfill that will be provided by the
unicast bursts and any payload-specific information MUST NOT be
smaller than the specified value. Otherwise, the backfill will
not be able to build up the desired level of buffer at the RTP_Rx
and can cause buffer underruns.
Type: 2
o Max RAMS Buffer Fill Requirement (32 bits): Optional TLV element
that denotes the maximum milliseconds of data that the RTP_Rx can
buffer without losing the data due to buffer overflow.
The RTP_Rx can have knowledge of its buffering requirements.
These requirements can be application or device specific. For
instance, one particular RTP_Rx might have more physical memory
than another RTP_Rx and thus can buffer more data. If the BRS
knows the buffering ability of the receiver, the BRS can tailor
the burst(s) more precisely. The methods used by the receiver to
determine this value are application specific and thus out of the
scope of this document.
If specified, the amount of backfill that will be provided by the
unicast bursts and any payload-specific information MUST NOT be
larger than this value. Otherwise, the backfill may cause buffer
overflows at the RTP_Rx.
Type: 3
o Max Receive Bitrate (64 bits): Optional TLV element that denotes
the maximum bitrate (in bits per second) at which the RTP_Rx can
process the aggregation of the unicast burst(s) and any payload-
specific information that will be provided by the BRS. The limits
can include local receiver limits as well as network limits that
are known to the receiver.
If specified, the total bitrate of the unicast burst(s) plus any
payload-specific information MUST NOT be larger than this value.
Otherwise, congestion and packet loss are more likely to occur.
Type: 4
o Preamble-only Allowed (0 bits): Optional TLV element that
indicates that the RTP_Rx is willing to receive only the preamble
information for the desired primary multicast stream(s) in case
the BRS cannot send the unicast burst stream(s). (In such cases,
the BRS will not accept the request, but will send a response code
511 in the RAMS-I message as defined in Section 11.6.) Note that
Ver Steeg, et al. Standards Track [Page 33]

RFC 6285 RAMS June 2011
the RTP_Rx signals the particular preamble format(s) it supports
through a public or a private extension in the same RAMS-R
message.
If this TLV element does not exist in the RAMS-R message, the BRS
MUST NOT respond to the RAMS-R message by sending the preamble
information only. Since this TLV element does not carry a Value
field, the Length field MUST be set to zero.
Type: 5
o Supported Enterprise Number(s): Optional TLV element that
indicates the enterprise number(s) that the RTP_Rx implementation
supports. Similar to the private extensions, the enterprise
numbers here are as listed on http://www.iana.org. This TLV
element, if exists, provides a hint to the BRS about which private
extensions the RTP_Rx can potentially support. Note that an
RTP_Rx does not necessarily support all the private extensions
under a particular enterprise number. Unless the BRS explicitly
knows which private extensions an RTP_Rx supports (through this or
some out-of-band means), the BRS SHOULD NOT use private extensions
in the RAMS-I messages. The BRS SHOULD rather use only vendor-
neutral extensions. The Length field of this TLV element is set
to 4*n, where n is the number of enterprise number entries.
Type: 6
7.3. RAMS Information
The RAMS Information (RAMS-I) message is identified by SFMT=2. This
message is used to describe the unicast burst that will be sent for
rapid acquisition. It also includes other useful information for the
RTP_Rx as described below.
A separate RAMS-I message with the appropriate response code is sent
in the unicast burst RTP session by the BRS for each primary
multicast stream that has been requested by the RTP_Rx. In each of
these RAMS-I messages, the media sender SSRC and packet sender SSRC
fields are both set to the SSRC of the BRS, which equals the SSRC of
the respective primary multicast stream.
The FCI field MUST contain only one RAMS Information message. The
FCI field has the structure depicted in Figure 8.
The semantics of the RAMS-I message is independent of the payload
type carried in the primary multicast RTP session.
Ver Steeg, et al. Standards Track [Page 34]

RFC 6285 RAMS June 2011
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SFMT=2 | MSN | Response |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Optional TLV-encoded Fields (and Padding, if needed) :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: FCI Field Syntax for the RAMS Information Message
A RAMS-I message has the following fields:
o Message Sequence Number (MSN) (8 bits) : Mandatory field that
denotes the sequence number of the RAMS-I message for the
particular media sender SSRC specified in the message header. The
MSN value SHALL be set to zero when a new RAMS request is
received. During rapid acquisition, the same RAMS-I message MAY
be repeated for redundancy purposes without incrementing the MSN
value. If an updated RAMS-I message will be sent (either with new
information or updated information), the MSN value SHALL be
incremented by one. In the MSN field, the regular wrapping rules
apply. Note that if the RTP_Rx has sent an updated request, it
MUST check every RAMS-I message entirely, regardless of whether or
not the MSN value has changed.
o Response (16 bits): Mandatory field that denotes the response
code for this RAMS-I message. This document defines several
initial response codes in Section 7.3.1 and registers them with
IANA in Section 11.6. If a new vendor-neutral response code will
be defined, it MUST be registered with IANA through the guidelines
specified in Section 11.6. If the new response code is intended
to be used privately by a vendor, there is no need for IANA
management. Instead, the vendor MUST use the private extension
mechanism (Section 7.1.2) to convey its message and MUST indicate
this by putting zero in the Response field.
When the RTP_Rx receives a RAMS-I message with a response code
that it does not understand, the RTP_Rx MUST send a RAMS-T message
immediately to the BRS.
The following TLV elements have been defined for the RAMS-I messages:
o Media Sender SSRC (32 bits): Optional TLV element that specifies
the media sender SSRC of the unicast burst stream. If the FT_Ap
that received the RAMS-R message is associated with a single
primary multicast stream but the requested media sender SSRC does
not match the SSRC of the RTP stream associated with this FT_Ap,
Ver Steeg, et al. Standards Track [Page 35]

RFC 6285 RAMS June 2011
the BRS includes this TLV element in the initial RAMS-I message to
let the RTP_Rx know that the media sender SSRC has changed. If
the two SSRCs match, there is no need to include this TLV element.
Type: 31
o RTP Seqnum of the First Packet (16 bits): TLV element that
specifies the RTP sequence number of the first packet that will be
sent in the respective unicast RTP stream. This allows the RTP_Rx
to know whether one or more packets sent by the BRS have been
dropped at the beginning of the stream. If the BRS accepts the
RAMS request, this element exists. If the BRS rejects the RAMS
request, this element SHALL NOT exist.
Type: 32
o Earliest Multicast Join Time (32 bits): TLV element that
specifies the delta time (in ms) between the arrival of the first
RTP packet in the unicast RTP stream (which could be a burst
packet or a payload-specific packet) and the earliest time instant
when an RTP_Rx MAY send an SFGMP Join message to join the
multicast session. A zero value indicated in this element means
that the RTP_Rx MAY send the SFGMP Join message right away. If
the RTP_Rx does not want to wait until the earliest multicast join
time, it MAY send a RAMS-T message, and after a reasonable amount
of time, it MAY join the SSM session.
If the RAMS request has been accepted, the BRS sends this element
at least once so that the RTP_Rx knows when to join the multicast
session. If the burst request has been rejected as indicated in
the Response field, this element MUST indicate a zero value. In
that case, it is up to the RTP_Rx when or whether to join the
multicast session.
When the BRS serves two or more bursts and sends a separate RAMS-I
message for each burst, the join times specified in these RAMS-I
messages SHOULD correspond to more or less the same time instant,
and the RTP_Rx sends the SFGMP Join message based on the earliest
join time.
Type: 33
o Burst Duration (32 bits): Optional TLV element that denotes the
time the burst will last (in ms), i.e., the difference between the
expected transmission times of the first and the last burst
packets that the BRS is planning to send in the respective unicast
RTP stream. In the absence of additional stimulus, the BRS will
Ver Steeg, et al. Standards Track [Page 36]

RFC 6285 RAMS June 2011
send a burst of this duration. However, the burst duration can be
modified by subsequent events, including changes in the primary
multicast stream and reception of RAMS-T messages.
The BRS MUST terminate the flow in the timeframe indicated by this
burst duration or the duration established by those subsequent
events even if it does not get a RAMS-T or a BYE message from the
RTP_Rx. It is OPTIONAL to send this element in a RAMS-I message
when the burst request is accepted. If the burst request has been
rejected as indicated in the Response field, this element MAY be
omitted or indicate a zero value.
Type: 34
o Max Transmit Bitrate (64 bits): Optional TLV element that denotes
the maximum bitrate (in bits per second) that will be used by the
BRS for the RTP stream associated with this RAMS-I message.
Type: 35
7.3.1. Response Code Definitions
1xx-Level Response Codes: These response codes are sent for
informational purposes.
o 100: This is used when the BRS wants to update a value that was
sent earlier to the RTP_Rx.
2xx-Level Response Codes: These response codes are sent to indicate
success.
o 200: This is used when the server accepts the RAMS request.
o 201: This is used when the unicast burst has been completed and
the BRS wants to notify the RTP_Rx.
4xx-Level Response Codes: These response codes are sent to indicate
that the message sent by the RTP_Rx is either improperly formatted or
contains an invalid value. The rules the RTP_Rx needs to follow upon
receiving one of these response codes are outlined in Step 5 in
Section 6.2. The 4xx-level response codes are also used as status
codes in the Multicast Acquisition Report Block [MULTICAST-ACQ] in
order to collect RTP_Rx-based error conditions.
o 400: This is used when the RAMS-R message is improperly
formatted.
Ver Steeg, et al. Standards Track [Page 37]

RFC 6285 RAMS June 2011
o 401: This is used when the minimum RAMS buffer fill requirement
value indicated in the RAMS-R message is invalid.
o 402: This is used when the maximum RAMS buffer fill requirement
value indicated in the RAMS-R message is invalid.
o 403: This is used when the maximum receive bitrate value
indicated in the RAMS-R message is insufficient.
o 404: This is used when the RAMS-T message is improperly
formatted.
5xx-Level Response Codes: These response codes are sent to indicate
an error on the BRS side. The rules the RTP_Rx needs to follow upon
receiving one of these response codes are outlined in Step 3 in
Section 6.2. The 5xx-level response codes are also used as status
codes in the Multicast Acquisition Report Block [MULTICAST-ACQ] in
order to collect BRS-based error conditions.
o 500: This is used when the BRS has experienced an internal error
and cannot accept the RAMS request.
o 501: This is used when the BRS does not have enough bandwidth to
send the unicast burst stream.
o 502: This is used when the BRS terminates the unicast burst
stream due to network congestion.
o 503: This is used when the BRS does not have enough CPU resources
to send the unicast burst stream.
o 504: This is used when the BRS does not support sending a unicast
burst stream.
o 505: This is used when the requesting RTP_Rx is not eligible to
receive a unicast burst stream.
o 506: This is used when RAMS functionality is not enabled for the
requested multicast stream.
o 507: This is used when the BRS cannot find a valid starting point
for the unicast burst stream that satisfies the RTP_Rx's
requirements.
o 508: This is used when the BRS cannot find the essential
reference information for the requested multicast stream.
Ver Steeg, et al. Standards Track [Page 38]

RFC 6285 RAMS June 2011
o 509: This is used when the BRS cannot match the requested SSRC to
any of the streams it is serving.
o 510: This is used when the BRS cannot serve the requested entire
session.
o 511: This is used when the BRS sends only the preamble
information but not the whole unicast burst stream.
o 512: This is used when the RAMS request is denied due to a policy
for the requested multicast stream, the RTP_Rx, or this particular
BRS.
7.4. RAMS Termination
The RAMS Termination (RAMS-T) message is identified by SFMT=3.
The RAMS Termination is used to assist the BRS in determining when to
stop the burst. A separate RAMS-T message is sent in the unicast
burst RTP session by the RTP_Rx for each primary multicast stream
that has been served by the BRS. Each of these RAMS-T message's
media sender SSRC field SHALL BE populated with the SSRC of the media
stream to be terminated. If the media sender SSRC populated in the
RAMS-T message does not match the SSRC of the burst served by the
BRS, BRS SHALL ignore the RAMS-T message.
If the RTP_Rx wants the BRS to stop a burst prematurely, it sends a
RAMS-T message as described below. Upon receiving this message, the
BRS stops the respective burst immediately. If the RTP_Rx wants the
BRS to terminate all of the bursts, it needs to send all of the
respective RAMS-T messages in a single compound RTCP packet.
The default behavior for the RTP_Rx is to send a RAMS-T message
immediately after it joined the multicast session and started
receiving multicast packets. In that case, the RTP_Rx includes the
sequence number of the first RTP packet received in the primary
multicast stream in the RAMS-T message. With this information, the
BRS can decide when to terminate the unicast burst.
The FCI field MUST contain only one RAMS Termination. The FCI field
has the structure depicted in Figure 9.
The semantics of the RAMS-T message is independent of the payload
type carried in the primary multicast RTP session.
Ver Steeg, et al. Standards Track [Page 39]

RFC 6285 RAMS June 2011
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SFMT=3 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Optional TLV-encoded Fields (and Padding, if needed) :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: FCI field syntax for the RAMS Termination message
o Extended RTP Seqnum of First Multicast Packet (32 bits): Optional
TLV element that specifies the extended RTP sequence number of the
first packet received from the SSM session for a particular
primary multicast stream. The low 16 bits contain the sequence
number of the first packet received from the SSM session, and the
most significant 16 bits extend that sequence number with the
corresponding count of sequence number cycles, which can be
maintained according to the algorithm in Appendix A.1 of
[RFC3550].
Type: 61
8. SDP Signaling8.1. Definitions
The syntax of the 'rtcp-fb' attribute has been defined in [RFC4585].
Here we add the following syntax to the 'rtcp-fb' attribute (the
feedback type and optional parameters are all case sensitive):
(In the following ABNF [RFC5234], rtcp-fb-nack-param is used as
defined in [RFC4585].)
rtcp-fb-nack-param =/ SP "rai"
The following parameter is defined in this document for use with
'nack':
o 'rai' stands for Rapid Acquisition Indication and indicates the
use of RAMS messages as defined in Section 7.
This document also defines a new media-level SDP attribute
('rams-updates') that indicates whether or not the BRS supports
updated request messages. This attribute is used in a declarative
manner and no Offer/Answer Model behavior is specified. If the BRS
supports updated request messages and this attribute is included in
the SDP description, the RTP_Rx can send updated requests. The BRS
might or might not be able to accept value changes in every field in
Ver Steeg, et al. Standards Track [Page 40]

RFC 6285 RAMS June 2011
an updated RAMS-R message. However, if the 'rams-updates' attribute
is not included in the SDP description, the RTP_Rx SHALL NOT send
updated requests. The RTP_Rx MAY still repeat its initial request
without changes, though.
8.2. Requirements
The use of SDP to describe the RAMS entities normatively requires
support for:
o The SDP grouping framework and flow identification (FID) semantics
[RFC5888]
o The RTP/AVPF profile [RFC4585]
o The RTP retransmission payload format [RFC4588]
o The RTCP extensions for SSM sessions with unicast feedback
[RFC5760]
o The 'multicast-rtcp' attribute [RFC6128]
o Multiplexing RTP and RTCP on a single port on both endpoints in
the unicast session [RFC5761]
o The 'portmapping-req' attribute [RFC6284]
Support for the source-specific media attributes [RFC5576] can also
be needed when the 'ssrc' attribute is to be used for the media
descriptions.
8.3. Example and Discussion
This section provides a declarative SDP [RFC4566] example (for the
RTP_Rx side) for enabling rapid acquisition of multicast RTP
sessions.
v=0
o=ali 1122334455 1122334466 IN IP4 rams.example.com
s=Rapid Acquisition Example
t=0 0
a=group:FID 1 2
a=rtcp-unicast:rsi
m=video 41000 RTP/AVPF 98
i=Primary Multicast Stream
c=IN IP4 233.252.0.2/255
a=source-filter:incl IN IP4 233.252.0.2 198.51.100.1
a=rtpmap:98 MP2T/90000
Ver Steeg, et al. Standards Track [Page 41]

RFC 6285 RAMS June 2011
a=multicast-rtcp:42000
a=rtcp:43000 IN IP4 192.0.2.1
a=rtcp-fb:98 nack
a=rtcp-fb:98 nack rai
a=ssrc:123321 cname:iptv-ch32@rams.example.com
a=rams-updates
a=portmapping-req:54000 IN IP4 192.0.2.1
a=mid:1
m=video 51000 RTP/AVPF 99
i=Unicast Retransmission Stream (Ret. and Rapid Acq. Support)
c=IN IP4 192.0.2.1
a=sendonly
a=rtpmap:99 rtx/90000
a=rtcp-mux
a=rtcp:51500
a=fmtp:99 apt=98;rtx-time=5000
a=portmapping-req:55000
a=mid:2
Figure 10: Example SDP for a Single-Channel RAMS Scenario
In this example SDP description, we have a primary multicast (source)
stream and a unicast retransmission stream. The source stream is
multicast from a distribution source (with a source IP address of
198.51.100.1) to the multicast destination address of 233.252.0.2 and
port 41000. The corresponding RTCP traffic is multicast to the same
multicast destination address at port 42000. Here, we are assuming
that the multicast RTP and RTCP ports are carefully chosen so that
different RTP and RTCP streams do not collide with each other.
The feedback target (FT_Ap) residing on the RS (with an address of
192.0.2.1) at port 43000 is declared with the "a=rtcp" line
[RFC3605]. The support for the conventional retransmission is
indicated through the "a=rtcp-fb:98 nack" line. The RTP receiver(s)
can report missing packets on the source stream to the feedback
target and request retransmissions. The SDP above includes the
"a=sendonly" line for the media description of the retransmission
stream since the retransmissions are sent in only one direction.
The support for rapid acquisition is indicated through the "a=rtcp-
fb:98 nack rai" line. The parameter 'rtx-time' applies to both the
conventional retransmissions and rapid acquisition. However, when
rapid acquisition is enabled, the standard definition for the
parameter 'rtx-time' given in [RFC4588] is not followed. Instead,
when rapid acquisition support is enabled, 'rtx-time' specifies the
time in milliseconds that the BRS keeps an RTP packet in its cache
Ver Steeg, et al. Standards Track [Page 42]

RFC 6285 RAMS June 2011
available for retransmission (measured from the time the packet was
received by the BRS, not from the time indicated in the packet
timestamp).
Once an RTP_Rx has acquired an SDP description, it can ask for rapid
acquisition before it joins a primary multicast RTP session. To do
so, it sends a RAMS-R message to the feedback target of that primary
multicast RTP session. If the FT_Ap is associated with only one RTP
stream, the RTP_Rx does not need to learn the SSRC of that stream via
an out-of-band method. If the BRS accepts the rapid acquisition
request, it will send a RAMS-I message with the correct SSRC
identifier. If the FT_Ap is associated with a multi-stream RTP
session and the RTP_Rx is willing to request rapid acquisition for
the entire session, the RTP_Rx again does not need to learn the SSRCs
via an out-of-band method. However, if the RTP_Rx intends to request
a particular subset of the primary multicast streams, it must learn
their SSRC identifiers and list them in the RAMS-R message. Since
this RTP_Rx has not yet received any RTP packets for the primary
multicast stream(s), the RTP_Rx must in this case learn the SSRC
value(s) from the 'ssrc' attribute of the media description
[RFC5576]. In addition to the SSRC value, the 'cname' source
attribute must also be present in the SDP description [RFC5576].
Listing the SSRC values for the primary multicast streams in the SDP
file does not create a problem in SSM sessions when an SSRC collision
occurs. This is because in SSM sessions, an RTP_Rx that observed an
SSRC collision with a media sender must change its own SSRC [RFC5760]
by following the rules defined in [RFC3550].
A feedback target that receives a RAMS-R message becomes aware that
the RTP_Rx wants to rapidly catch up with a primary multicast RTP
session. If the necessary conditions are satisfied (as outlined in
Section 7 of [RFC4585]) and available resources exist, the BRS can
react to the RAMS-R message by sending any transport-layer (and
optional payload-specific, when allowed) feedback message(s) and
starting the unicast burst.
In this section, we considered the simplest scenario where the
primary multicast RTP session carried only one stream and the RTP_Rx
wanted to rapidly acquire this stream only. Best practices for
scenarios where the primary multicast RTP session carries two or more
streams or the RTP_Rx wants to acquire one or more streams from
multiple primary multicast RTP sessions at the same time are
presented in [RAMS-SCENARIOS].
Ver Steeg, et al. Standards Track [Page 43]

RFC 6285 RAMS June 20119. NAT Considerations
For a variety of reasons, one or more Network Address Port
Translators (NAPT, hereafter simply called NAT) can exist between the
RTP_Rx and RS. NATs have a variety of operating characteristics for
UDP traffic [RFC4787]. For a NAT to permit traffic from the BRS to
arrive at the RTP_Rx, the NAT(s) must first either:
a. See UDP (or DCCP) traffic sent from the RTP_Rx (which is on the
'inside' of the NAT) to the BRS (which is on the 'outside' of the
NAT). This traffic has the same transport address as the
subsequent response traffic, or
b. Be configured to forward certain ports (e.g., using HTML
configuration or Universal Plug and Play (UPnP) Internet Gateway
Device (IGD) [UPnP-IGD]). Details of this are out of the scope
of this document.
For both (a) and (b), the RTP_Rx is responsible for maintaining the
NAT's state if it wants to receive traffic from the BRS on that port.
For (a), the RTP_Rx MUST send UDP traffic to keep the NAT binding
alive, at least every 30 seconds [RFC4787]. While (a) is more like
an automatic/dynamic configuration, (b) is more like a manual/static
configuration.
When the RTP_Rx sends a request (RAMS-R) message to the FT as unicast
feedback in the primary multicast RTP session and the request is
received by the FT, a new unicast burst RTP session will be
established between the BRS and RTP_Rx.
While the FT and BRS ports on the RS are already signaled via out-of-
band means (e.g., SDP), the RTP_Rx needs to convey the RTP and RTCP
ports it wants to use on its side for the new session. Since there
are two RTP sessions (one multicast and one unicast) involved during
this process and one of them is established upon a feedback message
sent in the other one, this requires an explicit port mapping method.
Applications using RAMS MUST support the method presented in
[RFC6284] both on the RS and RTP_Rx side to allow RTP receivers to
use their desired ports and to support RAMS behind NAT devices. The
port mapping message exchange needs to take place whenever the RTP_Rx
intends to make use of the RAMS protocol for rapidly acquiring a
specific multicast RTP session prior to joining the associated SSM
session.
Ver Steeg, et al. Standards Track [Page 44]

RFC 6285 RAMS June 201110. Security Considerations
Applications that are using RAMS make heavy use of the unicast
feedback mechanism described in [RFC5760], the payload format defined
in [RFC4588], and the port mapping solution specified in [RFC6284].
Thus, these applications are subject to the general security
considerations discussed in those documents. In particular, the
threats and risks discussed in [RFC5760] need to be considered
carefully. In this section, we give an overview of the guidelines
and suggestions described in these specifications from a RAMS
perspective. We also discuss the security considerations that
explicitly apply to applications using RAMS.
First of all, much of the session description information is
available in the SDP descriptions that are distributed to the media
senders, retransmission servers, and RTP receivers. Adequate
security measures are RECOMMENDED to ensure the integrity and
authenticity of the SDP descriptions so that transport addresses of
the media senders, distribution sources, feedback targets, and other
session-specific information can be protected. See [RFC4566] for
details.
Compared to an RTCP NACK message that triggers one or more
retransmissions, a RAMS Request (RAMS-R) message can trigger a new
burst stream to be sent by the retransmission server. Depending on
the application-specific requirements and conditions existing at the
time of the RAMS-R reception by the retransmission server, the
resulting burst stream can potentially contain a large number of
retransmission packets. Since these packets are sent faster than the
nominal rate, RAMS consumes more resources on the retransmission
servers, RTP receivers, and the network. In particular, this can
make denial-of-service (DoS) attacks more intense and hence more
harmful than attacks that target ordinary retransmission sessions.
As RAMS messages are sent as RTCP messages, counter-measures SHOULD
be taken to prevent tampered or spoofed RTCP packets, following the
suggestions given in [RFC4588]. Tampered RAMS-R messages can trigger
inappropriate burst streams or alter the existing burst streams in an
inappropriate way. For example, if the Max Receive Bitrate field is
altered by a tampered RAMS-R message, the updated burst can overflow
the buffer at the receiver side or, oppositely, can slow down the
burst to the point that it becomes useless. Tampered RAMS
Termination (RAMS-T) messages can terminate valid burst streams
prematurely resulting in gaps in the received RTP packets. RAMS
Information (RAMS-I) messages contain fields that are critical for a
successful rapid acquisition. Any tampered information in the RAMS-I
message can easily cause an RTP receiver to make wrong decisions.
Consequently, the RAMS operation can fail.
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RTCP BYE messages are similar to RAMS-T messages in the sense that
they can be used to stop an existing burst. The CNAME of an RTP
receiver is used to bind the RTCP BYE message to an existing burst.
Thus, one should be careful if the CNAMEs are reasonably easy to
guess and off-path attacks can be performed. Also note that the
CNAMEs might be redistributed to all participants in the multicast
group (as in ASM or the simple feedback model of [RFC5760]).
The retransmission server has to consider if values indicated in a
RAMS-R message are reasonable. For example, a request demanding a
large value of many seconds in the Min RAMS Buffer Fill Requirement
element should, unless special use cases exist, likely be rejected
since it is likely to be an attempt to prolong a DoS attack on the
retransmission server, RTP receiver, and/or the network. The Max
Receive Bitrate could also be set to a very large value to try to get
the retransmission server to cause massive congestion by bursting at
a bitrate that will not be supported by the network. An RTP_Rx
should also consider if the values for the Earliest Multicast Join
Time and Burst Duration indicated by the retransmission server in a
RAMS-I message are reasonable. For example, if the burst packets
stop arriving and the join time is still significantly far into the
future, this could be a sign of a man-in-the-middle attack where the
RAMS-I message has been manipulated by an attacker.
A basic mitigation against DoS attacks introduced by an attacker
injecting tampered RAMS messages is source address validation
[RFC2827]. Also, most of the DoS attacks can be prevented by the
integrity and authenticity checks enabled by Secure RTP (SRTP)
[RFC3711]. However, an attack can still be started by legitimate
endpoints that send several valid RAMS-R messages to a particular
feedback target in a synchronized fashion and in a very short amount
of time. Since a RAMS operation can temporarily consume a large
amount of resources, a series of the RAMS-R messages can temporarily
overload the retransmission server. In these circumstances, the
retransmission server can, for example, reject incoming RAMS requests
until its resources become available again. One means to ameliorate
this threat is to apply a per-endpoint policing mechanism on the
incoming RAMS requests. A reasonable policing mechanism should
consider application-specific requirements and minimize false
negatives.
In addition to the DoS attacks, man-in-the-middle and replay attacks
will also be harmful. RAMS-R messages do not carry any information
that allows the retransmission server to detect duplication or replay
attacks. Thus, the possibility of a replay attack using a captured
valid RAMS-R message exists unless a mitigation method such as Secure
RTCP (SRTCP) is used. Similarly, RAMS-T messages can be replayed.
The RAMS-I has a sequence number that makes replay attacks less
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RFC 6285 RAMS June 2011
likely but not impossible. Man-in-the-middle attacks that are
capable of capturing, injecting, or modifying the RAMS messages can
easily accomplish the attacks described above. Thus, cryptographic
integrity and authentication is the only reliable protection. To
protect the RTCP messages from man-in-the-middle and replay attacks,
the RTP receivers and retransmission server SHOULD perform a Datagram
Transport Layer Security (DTLS)-SRTP handshake [RFC5764] over the
RTCP channel. Because there is no integrity-protected signaling
channel between an RTP receiver and the retransmission server, the
retransmission server MUST maintain a list of certificates owned by
legitimate RTP receivers, or their certificates MUST be signed by a
trusted Certificate Authority. Once the DTLS-SRTP security is
established, non-SRTCP-protected messages received from a particular
RTP receiver are ignored by the retransmission server. To reduce the
impact of DTLS-SRTP overhead when communicating with different
feedback targets on the same retransmission server, it is RECOMMENDED
that RTP receivers and the retransmission server both support TLS
Session Resumption without Server-side State [RFC5077]. To help
scale SRTP to handle many RTP receivers asking for retransmissions of
identical data, implementors may consider using the same SRTP key for
SRTP data sent to the receivers [SRTP-EKT] and be aware that such key
sharing allows those receivers to impersonate the sender. Thus,
source address validation remains important.
[RFC4588] RECOMMENDS that cryptography mechanisms be used for the
retransmission payload format to provide protection against known
plain-text attacks. As discussed in [RFC4588], the retransmission
payload format sets the timestamp field in the RTP header to the
media timestamp of the original packet, and this does not compromise
the confidentiality. Furthermore, if cryptography is used to provide
security services on the original stream, then the same services,
with equivalent cryptographic strength, MUST be provided on the
retransmission stream per [RFC4588].
Finally, a retransmission server that has become subverted by an
attacker is almost impossible to protect against as such a server can
perform a large number of different actions to deny service to
receivers.
11. IANA Considerations
The following contact information is used for all registrations in
this document:
Ali Begen
abegen@cisco.com
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RFC 6285 RAMS June 2011
Note that the "RAMS" (value 2) in the Multicast Acquisition Method
Registry refers to the method described in Section 6 of this
document.
11.1. Registration of SDP Attributes
This document registers a new attribute name in SDP.
SDP Attribute ("att-field"):
Attribute name: rams-updates
Long form: Support for Updated RAMS Request Messages
Type of name: att-field
Type of attribute: Media level
Subject to charset: No
Purpose: See this document
Reference: [RFC6285]
Values: None
11.2. Registration of SDP Attribute Values
This document registers a new value for the 'nack' attribute to be
used with the 'rtcp-fb' attribute in SDP. For more information about
the 'rtcp-fb' attribute, refer to Section 4.2 of [RFC4585].
Value name: rai
Long name: Rapid Acquisition Indication
Usable with: nack
Reference: [RFC6285]
11.3. Registration of FMT Values
Within the RTPFB range, the following format (FMT) value is
registered:
Name: RAMS
Long name: Rapid Acquisition of Multicast Sessions
Value: 6
Reference: [RFC6285]
11.4. SFMT Values for RAMS Messages Registry
This document creates a new sub-registry for the sub-feedback message
type (SFMT) values to be used with the FMT value registered for RAMS
messages. The registry is called the SFMT Values for RAMS Messages
Registry. This registry is managed by the IANA according to the
Specification Required policy of [RFC5226].
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The length of the SFMT field in the RAMS messages is a single octet,
allowing 256 values. The registry is initialized with the following
entries:
Value Name Reference
----- -------------------------------------------------- ------------
0 Reserved [RFC6285]
1 RAMS Request [RFC6285]
2 RAMS Information [RFC6285]
3 RAMS Termination [RFC6285]
4-254 Unassigned - Specification Required
255 Reserved [RFC6285]
The SFMT values 0 and 255 are reserved for future use.
Any registration for an unassigned SFMT value needs to contain the
following information:
o Contact information of the one doing the registration, including
at least name, address, and email.
o A detailed description of what the new SFMT represents and how it
shall be interpreted.
New RAMS functionality is intended to be introduced by using the
extension mechanism within the existing RAMS message types not by
introducing new message types unless it is absolutely necessary.
11.5. RAMS TLV Space Registry
This document creates a new IANA TLV space registry for the RAMS
extensions. The registry is called the RAMS TLV Space Registry.
This registry is managed by the IANA according to the Specification
Required policy of [RFC5226].
The length of the Type field in the TLV elements is a single octet,
allowing 256 values. The Type values 0 and 255 are reserved for
future use. The Type values between (and including) 128 and 254 are
reserved for private extensions.
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